They would be great together, without a doubt. I use my Surface 3 and my Lumia 950XL and really love them both. Apps look and feel similar in both devices. Notifications from my phone was sent through Cortana to my Surface reliably. I can even project my phone screen (using Continuum) to my Surface and use it in Desktop-like mode when I am too lazy to send files between devices. Not to mention all Microsoft productivity apps would run smooth on both devices. You could try with Lumia 950/ 950XL if you are new to the platform since both flagships are now pretty cheap. But if you want the best performance (bar camera), you can give HP elite X3 a try. If you are in the US/ Canada, there is also Alcatel Idol 4S. Try them out, to me Windows 10 mobile and Surface devices are the perfect pair for each other.

I would keep the SE and just use Microsoft applications. Or an Android phone if you want better integration. (with Cortana, etc.)

Windows Phone complimenting Windows on the desktop is a good idea in theory but execution is middling.

This is my conclusion exactly based on personal experience. Using a Windows Phone to complete the all-Microsoft ecosystem on principle is fine, but in real life is less than ideal. Did you see Zac Bowden's review about his experience Using an iPhone in the Microsoft ecosystem?

They would be great together, without a doubt. I use my Surface 3 and my Lumia 950XL and really love them both. Apps look and feel similar in both devices. Notifications from my phone was sent through Cortana to my Surface reliably. I can even project my phone screen (using Continuum) to my Surface and use it in Desktop-like mode when I am too lazy to send files between devices.
Not to mention all Microsoft productivity apps would run smooth on both devices.
You could try with Lumia 950/ 950XL if you are new to the platform since both flagships are now pretty cheap. But if you want the best performance (bar camera), you can give HP elite X3 a try. If you are in the US/ Canada, there is also Alcatel Idol 4S.
Try them out, to me Windows 10 mobile and Surface devices are the perfect pair for each other.

Are you a marketing guy? because honestly there's no specific benefit of using a wp with a win10 device